Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Hellgrammites are the vicious larvae of the Dobsonflies, some of the only trout stream insects which pose a biting threat to the angler. The pincers of the adult are even more frightening that the larva's, and they're aggressive enough to use them once in a while.
This family's life cycle does not create good dry fly opportunities, but the larvae may be eaten by trout year-round. They are a secret told only by stomach samples of well-fed trout.
Gandoff on Mar 9, 2007March 9th, 2007, 2:02 pm EST
I have been fly fishing a lot over the past two years in the mountains of NC and Va. I have pumped the contents of many trout, and 2 out of 3 times, I find Hellgrammites in large brown trout, 18"+. perhaps they are not as numerous as caddis', but nether are crawfish, and wild trout like to eat them too!
As noted on several other replies, the warm bridge, and warm rocks, if you want to reasearch Hellgrammites, this is a good place to start.